When combined with a lens formed of a high-refractivity high-dispersion glass, a lens formed of a high-refractivity low-dispersion glass ensures that an optical system can be downsized while correcting chromatic aberration. It hence occupies an important place as an optical element constituting an image-sensing optical system and a projection optical system such as a projector, etc.
JP 2007-269584 A discloses such a high-refractivity low-dispersion glass. The glass disclosed in JP 2007-269584A has a refractive index nd of 1.75 to 2.00 and has a Ta2O5 content in the range of 0 to 25 mass %. However, all of the glasses that have a refractive index nd of at least 1.85 contain a large amount of Ta2O5. That is because it is indispensable to introduce a large amount of Ta2O5 for securing glass stability in the high refractivity region such as a refractive index nd of 1.75 or more. For such a high-refractivity low-dispersion glass, therefore, Ta2O5 is a main and essential component.